1. Field of the Invention
The present invention is directed to transmissions and, more particularly, to transmissions of the counter-rotating type used on airboats.
2. Description of the Background
Speed conversion is an important capability in the efficient utilization of rotary motive force. The need often arises for increasing or decreasing the speed of a driven member to a higher or lower speed, respectively, than that of a driving member. That is accomplished through the use of a transmission. Transmissions are found in various machines in which speed conversion is required. For example, in automobiles, a hydraulic transmission, with various combinations of gears, accomplishes the task of converting the high rotary speed of the gasoline engine to the lower rotational requirements of the driven axle. Typically, such transmissions are quite complex, requiring many parts to operate in synchronization, and are quite labor intensive for both assembly and service. Other machines in which speed conversion is necessary include water vessels and airboats. In water vessels and airboats, the ultimate driven member is a propeller. However, airboats may be provided with two propellers rotating in opposite directions, referred to as counter-rotating propellers. The transmission system for such a counter-rotating propeller system is typically more complex than a transmission system for a single propeller.
U.S. Pat. No. 5,807,149 entitled Airboat Systems and Methods for Increasing Engine Efficiency While Reducing Torque and Noise is one example of an airboat propulsion system in which a propeller is rotated by a hollow driven shaft. A further embodiment is provided wherein two propellers are rotated in opposite directions by counter-rotating coaxial hollow driven shafts. Other examples include U.S. Pat. No. 6,053,782 entitled Airboat Transmission, Lubrication System, and Associated Method and U.S. Pat. No. 5,724,867 entitled Airboat Transmission. In all three of those patents, the engine""s drive shaft is connected to the driven shafts through a series of gears.
Airboats are often powered by aircraft engines operating at approximately 2,500-3,000 revolutions per minute (rpm), but most use automobile engines that operate at 4,800 to 5,200 rpm. At those high speeds, using gears to couple the drive shaft of the engine to the driven shafts which carry the counter-rotating propellers places a high degree of stress on the gears and shafts. As a result, the gears can become locked up and shafts snap off, conditions that are difficult and expensive to repair. Thus, the need exists for a transmission system capable of reducing the rpm""s while efficiently coupling the torque of an automotive engine to the counter-rotating propellers of an airboat.
The present invention is directed to a counter-rotating, belt-driven transmission for use in connection with an airboat. The transmission comprises a frame having a front and a back. The front of the frame has an opening for receiving a drive shaft carrying a pulley while the back the frame has an opening for receiving a transmission assembly. The transmission assembly comprises a primary driven shaft carrying a pulley and a hollow, secondary, driven shaft coaxial with the primary driven shaft. A floating drive gear is carried by the primary driven shaft. Four idler gears are driven by the drive gear. A driven gear, carried by the secondary driven shaft, is driven by the idler gears to effect rotation of the secondary driven shaft in a direction opposite to the direction of rotation of the primary driven shaft. The belt drive of the present invention provides for an efficient coupling of the engine""s torque to the driven shafts while eliminating the possibility of lock-up of the transmission. The belt drive also eliminates the transmission of shock loads to the gears. The transmission assembly is sized to be a direct replacement for existing transmissions which have a single driven shaft and propeller. Those, and other advantages and benefits, will be apparent from the Description of the Preferred Embodiments herein below.